Knowledge of platelet function and erythrocyte aggregation in diabetes has derived largely from tests of aggregation by optical methods. The resulting data are empirical and difficult to relate to biochemical and morphologic parameters. Optical aggregometry also is unable to follow rates of reaction during the first 5 seconds of aggregation, a critical period in platelet function. These problems are in part obviated in the proposed research because of a sensitive new approach to erythrocyte aggregation and platelet function studies; namely stopped-flow aggregometry. The objectives are to characterize platelet function and erythrocyte aggregability in normal subjects, and in a large group of both insulin-dependent and insulin-independent diabetics. The basic principle of stopped-flow aggregometry is to react platelets or erythrocytes for very short but precisely controlled times. This allows true measurement of initial rates of aggregation. The technique to be used employs newly-developed modifications of resistive-particle counters which provide a continuous measurement of platelet or erythrocyte aggregation and of the initial "platelet shape change". Ancillary methods to be employed include scanning electron microscopy for visualizing the state of platelet and erythrocyte aggregation and isotopic measurements of the platelet release reaction. Knowledge of initial rates of reaction and the ability to correlate the various parameters measured by the ancillary methods promises to reveal new information about platelet function in both the normal and diabetic conditions. Since all subjects will be clinically evaluated, early detection of potential diabetic vascular complications may be possible.